Method of manufacturing expendable salt core for casting

ABSTRACT

A melt is made by heating a salt mixture containing a salt of sodium. The melt is set at a temperature higher than the liquidus temperature of the salt mixture, and poured into a mold for expendable core molding. The temperature when the melt is completely poured into the mold is set within a range not exceeding the liquidus temperature of the salt mixture by 30° C. An expendable salt core for casting is molded by solidifying the melt inside the mold. This makes it possible to more stably obtain the strength of a water-soluble expendable salt core for casting made of a salt cast product obtained by melting and molding salts of sodium and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing awater-soluble expendable salt core for casting.

2. Description of the Related Art

As is well known, casting such as aluminum die casting is a technique ofcasting a structure having a desired shape by injecting a melt of analuminum alloy into a metal mold at high speed and high pressure. Incasting like this, a core is used to mold a cast product having a hollowstructure, e.g., a water jacket for water cooling such as a cylinderblock of an internal combustion engine. A core used in a case like thisis apt to receive a large impact because a metal melt injected at highspeed from a gate impacts against the core. In addition, the castingpressure is high until the completion of solidification. Therefore, thecore is required to have strength that can withstand a high pressure andhigh temperature.

Also, as is well known, the core is removed from a cast product aftercasting. However, if a general sand expendable core solidified by aphenolic resin is used for a cast product having a complicated internalstructure, it is not easy to remove the expendable core. On the otherhand, water-soluble expendable salt cores removable by dissolution inhigh-temperature water or the like are disclosed in Japanese PatentPublication No. 48-039696, Japanese Patent Laid-Open No. 50-136225, andJapanese Patent Publication No. 52-010803. An expendable salt core ismanufactured by melting and molding a salt mixture of, e.g., sodiumcarbonate (Na₂CO₃), potassium chloride (KCl), and sodium chloride(NaCl), thereby obtaining a high pressure resistance, and improving theworkability and stability of casting.

As described above, an expendable salt core manufactured by melting andmolding a salt mixture and having a high strength has been developed.However, expendable salt cores have large variations in strength, andhence have not completely been put into practical use.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention solve the problems asdescribed above, and more stably obtain a practical strength of awater-soluble expendable salt core for casting made of a salt castproduct obtained by melting and molding salts of sodium and the like.

A method of manufacturing an expendable salt core for casting accordingto a preferred embodiment of the present invention includes the steps ofmaking a melt by heating a salt mixture containing a salt of sodium,setting a temperature of the melt at a temperature higher than aliquidus temperature of the salt mixture, and pouring the melt into amold for expendable core molding, and molding an expendable salt corefor casting by solidifying the melt inside the mold, wherein the pouringstep includes the step of setting, when the melt is completely pouredinto the mold, the temperature of the melt within a range not exceedingthe liquidus temperature of the salt mixture by 30° C.

In a preferred embodiment of the present invention, a melt of a saltmixture is heated to a temperature higher than the liquidus temperatureof the salt mixture and poured into a mold for expendable core molding,and the temperature of the melt when the pouring is complete is setwithin a range not exceeding the liquidus temperature of the saltmixture by 30° C. This makes it possible to more stably obtain thestrength of a water-soluble expendable salt core for casting made of asalt cast product obtained by melting and molding salts of sodium andthe like.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a cylinder block cast by using anexpendable salt core for casting according to a preferred embodiment ofthe present invention.

FIG. 2 is a photograph showing the result obtained by observing, with anelectron microscope, a polished surface of an expendable salt coremanufactured at a superheat of 10° C.

FIG. 3 is a photograph showing the result obtained by observing, with anelectron microscope, a polished surface of an expendable salt coremanufactured at a superheat of 40° C.

FIG. 4 is a photograph showing the result obtained by observing, with anelectron microscope, a fracture surface of an expendable salt coremanufactured at a superheat of 10° C.

FIG. 5 is a photograph showing the result obtained by observing, with anelectron microscope, a fracture surface of an expendable salt coremanufactured at a superheat of 40° C.

FIG. 6 is a graph showing the relationship between the superheat andstrength when melt pouring is complete.

FIG. 7 is a graph showing the relationship between the mixing ratio ofsodium chloride to sodium carbonate and the strength.

FIG. 8 is a side view of a specimen for use in bending strengthmeasurement.

FIG. 9 is a sectional view of the specimen shown in FIG. 8.

FIG. 10 is a view for explaining bending strength measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be explained belowwith reference to the accompanying drawings. First, the form of use ofan expendable salt core for casting according to a preferred embodimentof the present invention will be explained with reference to FIG. 1.Referring to FIG. 1, a cylinder block 101 is an engine cylinder blockmade of an aluminum alloy cast by using an expendable salt core 102 asthe expendable salt core for casting according to the present preferredembodiment. The cylinder block 101 is a part of a water-cooling,four-cycle, single-cylinder engine for a motorcycle, and molded into apredetermined shape by die casting.

The cylinder block 101 includes a cylinder bore 103, and a cylinder body104 including the cylinder bore 103. Although not shown, a crankcase isattached to the lower portion of the cylinder body 104. This crankcaseaxially supports a crankshaft via a bearing so that the crankshaft isrotatable.

The cylinder body 104 is a so-called closed deck type body. A waterjacket 106 is formed inside the cylinder body 104 by using theexpendable salt core 102. The water jacket 106 includes a cooling waterchannel formation portion (not shown), cooling water inlet (not shown),main cooling water channel 109, and communication channel 110. Thecooling water channel formation portion projects from one side portionof the cylinder body 104. The cooling water inlet is formed in thecooling water channel formation portion. The main cooling water channel109 is formed to communicate with a cooling water supply channel (notshown) formed inside of the cooling water channel formation portion, andcover the cylinder bore 103. The communication channel 110 extendsupward in FIG. 1 from the main cooling water channel 109, and opens in amating surface 104 a for a cylinder head (not shown) at the upper end ofthe cylinder body 104.

The water jacket 106 described above is formed to supply cooling waterflowing from the cooling water inlet to the main cooling water channel109 around the cylinder bore 103 through the cooling water supplychannel, and guide the cooling water from the main cooling water channel109 to an internal cooling water channel of the cylinder head throughthe communication channel 110. Since the water jacket 106 is thusformed, the cylinder body 104 is covered with the ceiling wall (the wallforming the mating surface 104 a) of the cylinder body 104, except thatthe communication channel 110 of the water jacket 106 opens in themating surface 104 a at the upper end to which the cylinder head is tobe connected, thereby constructing a closed deck type body.

The expendable salt core 102 for forming the water jacket 106 is formedinto a structure that integrally connects the individual portions of thewater jacket 106. To give a better understanding of the shape of theexpendable salt core 102 (the shape of the water jacket 106), FIG. 1depicts a state in which the cylinder body 104 is partially cut away.Note that reference numeral 111 denotes a camshaft driving chainpassage; and 112, a chain tensioner attaching hole.

The expendable salt core 102 according to this preferred embodiment ismanufactured by making a melt by heating a salt mixture containing asalt of sodium, raising the temperature of the melt to a hightemperature falling within a range not exceeding the liquidustemperature of the salt mixture by 30° C., pouring the melt into a moldfor expendable core molding, and molding the melt by solidifying itinside the mold. The method of manufacturing the expendable salt core102 will be described in detail later.

As shown in FIG. 1, the expendable salt core 102 is obtained byintegrally forming the cooling water channel formation portion formingthe cooling water inlet and cooling water supply channel, an annularportion 102 b having a shape surrounding the cylinder bore 103, and aplurality of projections 102 a projecting upward from the annularportion 102 b. The projections 102 a form the communication channel 110of the water jacket 106. As is conventionally well known, the expendablesalt core 102 is supported at a predetermined position inside a metalmold (not shown) by a core print (not shown) during die casting of thecylinder block 101, and removed by dissolution using hot water or vaporafter casting.

The expendable salt core 102 can be removed after casting by dipping thecylinder block 101 in a dissolving bath (not shown) containing adissolving liquid made of hydrochloric acid, hot water, and the like.When the cylinder block 101 is dipped in the dissolving liquid, thecooling water inlet of the cooling water channel formation portion ofthe expendable salt core 102 and the projections 102 a exposed in themating surface 104 a are brought into contact with the dissolvingsolution and dissolved. The dissolved portions gradually extend, and allportions are finally dissolved. In this expendable core moving step, hotwater or vapor may be sprayed with pressure from a hole, in order toaccelerate the dissolution of the expendable salt core 102 remaining inthe water jacket 106. In the expendable salt core 102, core prints canbe inserted, instead of the projections 102 a, in the prospectiveportions of the projections 102 a.

Also, carbonic acid gas is foamed when using hydrochloric acid in thestep of removing the expendable salt core 102 from the cylinder block101 as a cast product. Since a stirring action is obtained by thisfoaming, the dissolution can effectively be promoted. Furthermore, theexpendable salt core 102 contains sodium carbonate, and sodium carbonateshows alkaline properties when dissolved in water. An alkaline statelike this poses the problem that, e.g., the cylinder block 101 as analuminum cast product corrodes. The corrosion of the cylinder block canbe prevented by setting the pH close to 7 by adding hydrochloric acid.

The method of manufacturing the expendable salt core 102 will beexplained in detail below. The explanation will be made by taking a saltmixture obtained by mixing sodium chloride and sodium carbonate as anexample of the salt mixture containing a salt of sodium. In thispreferred embodiment, a salt mixture is first prepared by mixing sodiumchloride and sodium carbonate, and a melt of the salt mixture is made byheating the salt mixture to a temperature higher than the melting point.For example, a salt mixture (to be referred to as 30 mol % NaCl-70 mol %Na₂CO₃ hereinafter) is prepared by mixing 30 mol % of sodium chlorideand 70 mol % of sodium carbonate, and this salt mixture is heated to andheld at a temperature higher by about 50° C. to 80° C. than the liquidustemperature of the salt mixture, thereby making an entirely dissolvedmelt. As an example, the salt mixture described above need only beplaced in an alumina crucible and melted by an electric furnace. Notethat heating the above-mentioned salt mixture produces a molten saltcontaining sodium ion, chlorine ion, and carbonic acid ion.

The liquidus temperature includes a conventional liquidus temperature(experimental data used in microstructure control of materials, and aliquidus temperature (calculated data) calculated by thermodynamiccalculations from the thermodynamic data and mixing ratio of theconstituent materials of a salt mixture. The former experimental data isobtained by measuring a temperature at which a primary a crystal startsprecipitating when a salt mixture in a molten state is cooled. On theother hand, the latter calculated data is obtained by calculations by,e.g., “Thermo-Calc” by using thermodynamic data (see B. Sundman, B.Jansson, J.-O. Andresson, Calphad 9 (1985) 153. and Jun Yaokawa,Katsunari Oikawa and Koichi Anzai: “Thermodynamic Accessment ofKCl—K₂CO₃—NaCl—Na₂CO₃System”, CALPHAD, accepted (2007)). The liquidustemperature in this preferred embodiment is the latter calculated data.

Then, after the salt mixture contained in the crucible is completelymelted, the crucible is taken out from the electric furnace and cooledwith air. The cooling rate is 0.3° C. to 1.2° C. per sec. At the sametime, the salt mixture in the crucible is stirred at a rotational speedof three rotations per sec by using an alumina stirrer. The crucible iscooled while the salt mixture is thus stirred, and the melt of the saltmixture starts being poured into a metal mold when the temperature ofthe melt of the salt mixture is 758° C. higher by 15° C. than theliquidus temperature (743° C. for 30 mol % NaCl-70 mol % Na₂CO₃). Thatis, the temperature of the melt of the salt mixture is 758° C.immediately before the melt is poured into the metal mold. The metalmold is preheated to, e.g., about 100° C.

When the melt is poured into the metal mold, the melt is cooled to atemperature (753° C.) higher by 10° C. than the liquidus temperaturewhen pouring is complete, due to, e.g., the elapse of time to thecompletion of pouring and the absorption of heat to the metal mold. Inother words, the above-mentioned cooling is performed such that thetemperature of the melt when the melt is completely poured into themetal mold (when pouring is complete) is higher by 10° C. than theliquidus temperature. In this preferred embodiment, the temperature ofthe melt decreases by about 5° C. in the series of steps of pouring themelt into the metal mold. Note that in the following description, thedifference between the liquidus temperature and the temperature of themelt when pouring is complete, which is higher than the liquidustemperature, will be referred to as a superheat (superheat temperature).In the above-described case, the superheat is 10° C.

After that, an expendable salt core 102 is formed by solidifying themelt inside the metal mold. The expendable salt core 102 thus obtainedhas a high strength, i.e., the value of the bending strength exceeds 30MPa. Also, as shown in a scanning electron microscope (SEM) photographof FIG. 2, a fine granular primary a crystal (crystal grains) having aspindle shape is uniformly distributed in the solidified texture of theexpendable salt core 102. In addition, analysis by an energy dispersiveX-ray (EDX) diffractometer reveals that the crystal grains are made ofsodium carbonate.

On the other hand, as shown in FIG. 3, in a manufacturing method inwhich the same composition is used and the superheat is set at 40° C., adendritic crystal (dendrite microstructure) that presumably decreasesthe mechanical strength is observed as primary cells. Analysis by theEDX diffractometer reveals that this dendrite microstructure is alsomade of sodium carbonate.

When a fracture surface of the expendable salt core obtained by themanufacturing method in which the superheat is 10° C. is observed withthe SEM, the surface has a complicated three-dimensional structure asshown in FIG. 4. By contrast, when a fracture surface of the expendablesalt core obtained by the manufacturing method in which the superheat is40° C. is observed with the SEM, the surface is two-dimensionallycracked along the dendrite microstructure as shown in FIG. 5. Asdescribed above, the dendritic crystal grains (dendrite microstructure)readily grow to form giant crystal grains, and cleavage easily occurs inthese portions. This presumably decreases the strength. In thispreferred embodiment, a high strength is obtained probably because nosuch dendrite microstructure that decreases the strength is formed.

As shown in FIG. 6, a high strength as described above is perhapsobtainable as long as the superheat does not exceed 30° C. As shown inFIG. 6, the bending strength when the superheat exceeds 30° C. at thetime of completion of pouring is obviously lower than that when thesuperheat does not exceed 30° C. In the manufacturing method accordingto this preferred embodiment, therefore, the temperature width of thesuperheat is about 30° C., so the expendable salt core 102 can bemanufactured without strictly controlling the temperature and holding aconstant temperature. Note that FIG. 6 shows the results of measurementsof the strengths of expendable salt cores manufactured following thesame procedures as above by setting the mold temperature at 18° C. to53° C., 100° C., and 204° C. to 364° C. The mold temperature has littleeffect on the bending strength.

When manufacturing an expendable salt core by using a salt mixtureobtained by mixing sodium chloride and sodium carbonate, as shown inFIG. 7, if the superheat falls within a range (9° C. to 23° C.) notexceeding 30° C., a bending strength higher than that obtained by anyother superheat is obtained, regardless of the mixing ratio of sodiumchloride (NaCl) to sodium carbonate (Na₂CO₃). The highest strength isobtained when the mixing ratio is 1:1. Note that FIGS. 6 and 7 usenumerical values shown in Tables 1, 2, and 3 below. Note also that thevalue of 54.6 mol % NaCl-45.4 mol % Na₂CO₃ is obtained by thermodynamiccalculations by “Thermo-Calc” in the same manner as for the liquidustemperature.

TABLE 1 Liquidus Mold Bend- NaCl Na₂CO₃ Temper- Super- Temper- ingBending Sample Ratio Ratio ature heat ature Load Strength Number mol %mol % ° C. ° C. ° C. N MPa 1 100 0 801 10 100 399 3.3 2 90 10 766 9 1001933 16.1 3 90 10 766 9 100 902 7.5 4 90 10 766 10 100 1436 12.0 5 90 10766 10 100 1507 12.6 6 90 10 766 55 100 1177 9.8 7 80 20 731 9  9 254721.2 8 80 20 731 9  9 2766 23.1 9 80 20 731 9 100 2766 23.1 10 80 20 73110 100 2327 19.4 11 80 20 731 30 100 2259 18.8 12 80 20 731 62 100 170014.2 13 70 30 694 10 100 3194 26.6 14 70 30 694 14 100 2381 19.8 15 7030 694 14 100 2458 20.5 16 70 30 694 14 100 2260 18.8 17 70 30 694 14100 2157 18.0 18 70 30 694 30 100 2663 22.2 19 70 30 694 59 100 255721.3 20 60 40 654 10 100 2826 23.6 21 60 40 654 10 100 1364 11.4 22 6040 654 16 100 1412 11.8 23 60 40 654 16 100 2388 19.9 24 60 40 654 16100 1606 13.4 25 60 40 654 30 100 1315 11.0 26 60 40 654 30 100 798 6.627 60 40 654 56 100 1379 11.5 28 60 40 654 100 100 487 4.1 29 54.6 45.4632 10 100 3751 31.3 30 54.6 45.4 632 10 100 2482 20.7 31 54.6 45.4 63230 100 1996 16.6 32 54.6 45.4 632 30 100 2109 17.6 33 54.6 45.4 632 50100 1618 13.5 34 54.6 45.4 632 160 100 1749 14.6 35 50 50 654 10 1003442 28.7 36 50 50 654 10 100 4270 35.6 37 50 50 654 10 100 4632 38.6 3850 50 654 10 100 5087 42.4 39 50 50 654 30 100 2718 22.6 40 50 50 654 30100 2892 24.1

TABLE 2 Liquidus Mold Bend- NaCl Na₂CO₃ Temper- Super- Temper- ingBending Sample Ratio Ratio ature heat ature Load Strength Number mol %mol % ° C. ° C. ° C. N MPa 41 50 50 654 31 100 3188 26.6 42 50 50 654 31100 2795 23.3 43 50 50 654 31 100 2619 21.8 44 50 50 654 31 100 325027.1 45 50 50 654 50 100 2482 20.7 46 50 50 654 90 100 3438 28.6 47 5050 654 100 100 3245 27.0 48 40 60 700 10 100 3332 27.8 49 40 60 700 10100 3439 28.7 50 40 60 700 10 100 3347 27.9 51 40 60 700 23 100 341328.4 52 40 60 700 23 100 2790 23.2 53 40 60 700 23 100 2442 20.4 54 4060 700 30 100 2730 22.8 55 40 60 700 30 100 2773 23.~ 56 40 60 700 30100 2648 22.1 57 40 60 700 50 100 2367 19.7 58 40 60 700 100 100 203116.9 59 40 60 700 100 100 2737 22.8 60 30 70 743 10 18 3991 33.3 61 3070 743 10 100 3469 28.9 62 30 70 743 10 100 3519 29.3 63 30 70 743 10100 3552 29.6 64 30 70 743 10 204 4628 38.6 65 30 70 743 10 301 420935.1 66 30 70 743 20 100 3885 32.4 67 30 70 743 20 100 3904 32.5 68 3070 743 20 100 4021 33.5 69 30 70 743 20 100 3591 29.9 70 30 70 743 20314 2895 24.1 71 30 70 743 30 18 2679 22.3 72 30 70 743 30 100 2755 23.073 30 70 743 30 100 2616 21.8 74 30 70 743 30 100 2620 21.8 75 30 70 74330 300 3081 25.7 76 30 70 743 40 18 2218 18.5 77 30 70 743 40 100 218518.2 78 30 70 743 40 288 2473 20.6 79 30 70 743 50 18 2661 22.2 80 30 70743 50 100 2717 22.6

TABLE 3 Liquidus Mold Bend- NaCl Na₂CO₃ Temper- Super- Temper- ingBending Sample Ratio Ratio ature heat ature Load Strength Number mol %mol % ° C. ° C. ° C. N MPa 81 30 70 743 50 294 3009 25.1 82 30 70 743 6020 2269 18.9 83 30 70 743 60 102 2521 21.0 84 30 70 743 60 293 2080 17.385 30 70 743 70 99 2299 19.2 86 30 70 743 70 289 2295 19.1 87 30 70 74370 298 2215 18.5 88 30 70 743 80 96 2367 19.7 89 30 70 743 80 298 291824.3 90 30 70 743 85 326 1694 14.1 91 30 70 743 90 44 2410 20.1 92 30 70743 90 44 2243 18.7 93 30 70 743 100 53 1805 15.0 94 30 70 743 100 1001983 16.5 95 30 70 743 100 196 2345 19.5 96 30 70 743 100 364 1019 8.597 20 80 783 0 100 2198 18.3 98 20 80 783 10 100 2971 24.8 99 20 80 78310 100 1953 16.3 100 20 80 783 23 100 2156 18.0 101 20 80 783 30 1001265 10.5 102 20 80 783 30 100 2069 17.2 103 10 90 821 10 100 1243 10.4104 10 90 821 10 100 1379 11.5 105 10 90 821 10 100 2294 19.1 106 10 90821 16 100 1081 9.0 107 10 90 821 16 100 629 5.2 108 10 90 821 30 1001050 8.7 109 0 100 858 10 100 347 2.9

In this preferred embodiment as explained above, a melt is made byheating a salt mixture containing a salt of sodium, and this melt isheated to a temperature higher than the liquidus temperature of the saltmixture, poured into a mold for expendable core molding, and solidifiedinside the mold, thereby molding an expendable salt core for casting. Inparticular, the temperature of the melt when the melt is completelypoured into the mold is set within a range not exceeding the liquidustemperature of the salt mixture by 30° C. Consequently, a higher bendingstrength can be obtained as described previously. This makes it possibleto more stably obtain a practical strength of the expendable salt core(expendable salt core for casting). For example, even when the strengthvaries, the range of the variation falls inside a practical strengthrange.

The measurement of the bending strength will now be explained. In themeasurement of the bending strength, a square-pillar-like specimenhaving predetermined dimensions is formed, a load is applied on thespecimen, and a bending load is obtained from a maximum load required tobreak the specimen. First, the formation of the specimen will beexplained. A bar-like specimen 801 as shown in FIGS. 8 and 9 is formedby using a predetermined metal mold. The metal mold used is made of,e.g., chromium molybdenum steel such as SCM440H. FIG. 8 shows riserparts 802 used to fill the metal mold with a semi-solidified melt, butthe parts 802 are cut off in the measurement of the bending strength.Note that FIG. 8 is a side view, FIG. 9 is a sectional view taken alonga line b-b in FIG. 8, and the dimensions shown in FIGS. 8 and 9 are thedesign values of the metal mold.

The bending strength of the bar-like specimen 801 formed as describedabove is measured as shown in FIG. 10. First, the specimen 801 issupported by two support members 1001 arranged to form a space of 50 mmin a central portion of the specimen 801. In this state, in anintermediate portion between the two support members 1001, two loadingportions 1002 spaced apart by 10 mm apply a load on the specimen 801.The load applied on the specimen 801 is gradually increased, and a loadwhen the specimen 801 is broken is regarded as the bending load shown inTable 1.

A bending strength σ (MPa) can be calculated by an equation “σ=3LP/BH²”from a bending load P. In this equation, H indicates the length of thesection of the specimen in the loading direction, B indicates the lengthof the section of the specimen in a direction perpendicular to theloading direction, and L indicates the distance from the support member1001 as a fulcrum to the loading portion 1002 that applies a load. Thespecimen 801 is formed by pouring a melt in a solid-liquid coexistingstate into a metal mold. However, it is difficult to form a specimenhaving neither a flow line nor a shrinkage cavity and having a shapecompletely matching the mold dimensions. Therefore, the bending strengthis calculated by approximating the section of the specimen to an oblong,and assuming that H≈20 mm, B≈18 mm, and L=20 mm. By this approximation,the strength is estimated to be lower by about 0% to 20% than the actualstrength. For example, a specimen that breaks with a bending load of1,200 N can be regarded as stronger than an ideal specimen having abending strength of 10 MPa.

Note that various preferred embodiments of the present invention arealso applicable to a method of molding an expendable salt core by diecasting. Even when using die casting, the same effect as described abovecan be obtained as long as the superheat does not exceed 30° C. when amelt is completely poured into a mold (when melt injection into the moldis complete).

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

The invention claimed is:
 1. A method of manufacturing an expendablesalt core for casting, comprising the steps of: making a melt of a saltmixture containing at least two salts including a salt of sodium byheating the salt mixture; setting a temperature of the melt at atemperature higher than a liquidus temperature of the salt mixture, andpouring the melt into a mold for expendable core molding; and molding anexpendable salt core for casting by solidifying the melt inside themold; wherein the step of pouring the melt into a mold includes the stepof setting, when the melt is completely poured into the mold, thetemperature of the melt within a range higher by not less than 9° C.than the liquidus temperature of the salt mixture and not exceeding theliquidus temperature of the salt mixture by 30° C.
 2. A method ofmanufacturing an expendable salt core for casting according to claim 1,wherein the step of making a melt includes the step of heating amaterial obtained by mixing sodium chloride and sodium carbonate, as thesalt mixture.
 3. A method of manufacturing an expendable salt core forcasting according to claim 1, wherein the step of making a melt includesthe step of producing a molten salt containing sodium ion, chlorine ion,and carbonic acid ion, by heating the salt mixture.
 4. A method ofmanufacturing an expendable salt core for casting according to claim 1,wherein when the melt is completely poured into the mold, thetemperature of the melt is higher by 9° C. to 23° C. than the liquidustemperature of the salt mixture.